1. Field of the Invention
The present invention relates generally to systems and methods for designing and fabricating conductive wiring harnesses. More particularly, the invention relates to methods for fabricating wiring harnesses as uninterrupted (i.e., seamless) flexible printed circuitry of unlimited dimension on a continuous, web-fed substrate.
2. State of the Art
Presently, round wire harnesses (RWHs) are used in large scale systems (e.g., large appliances, automobiles, office wiring, industrial control panels, and commercial and military aircraft) to interconnect circuit components. An RWH typically includes a bundle of individual wire conductors terminated in connectors for attachment to the various circuit components.
While an RWH provides large scale interconnection of components, it suffers from significant drawbacks. For example, because RWH is used in large scale systems, the number of conductive paths is relatively large. The complex routing and connection of the various conductive paths therefore make it difficult to handle the RWH and render assembly of a system using RWH labor intensive. Use of an RWH also results in a highly disorganized wiring scheme in which misconnections of conductors frequently occur and in which location of faults (e.g., breaks in conductive paths) cannot be readily identified and corrected.
RWH includes a large number of individual, round conductors which consume a significant volume of space and which add substantial weight to a system. Because individual wires are routed, an RWH is also highly susceptible to cross-talk and to impedance mismatches with system components.
Further, because an RWH is a bundle of individual wire conductors, the strength of the overall system is limited to the strength of each individual conductor. Therefore, if a single wire is structurally stressed (i.e., typically the shortest conductor), that conductor can break. As mentioned previously, subsequent identification and correction of such breaks are difficult due to the routing complexity of an RWH. Further, this structural limitation often leads to a harness configuration being dictated by strength requirements rather than electrical requirements, such that the harness is often heavier than necessary.
Another significant drawback of an RWH is its inability to accommodate modifications in the conductive path layout. Rather, circuit component changes and/or routing modifications of the conductive paths are typically achieved by the addition of new conductive wiring paths, adding to the complexity of the overall system or by a complete redesign of the system, increasing the time required to produce a finished harness.
In small scale applications, efforts have been made to address the foregoing drawbacks with the introduction of flexible printed wiring, or FPW. For example, in the book entitled Handbook of Wiring, Cabling and Interconnecting for Electronics. 1972: McGraw-Hill, Chapter 9 relates to "Flexible Printed Wiring and Connector Systems". Flexible printed circuits are described therein which employ thin, flexible insulation to support circuitry which is covered with a thin overlay of insulation.
In a process schematic shown in Chapter 9, FIG. 10 of the aforementioned book, an FPW circuit layout is designed by first creating a circuit layout master. Using the circuit layout master, a layout of conductors is imaged on a photographic negative from which a camera positive is produced. A screen is then fabricated from the camera positive for applying resist to the copper surface of a base material (i.e., substrate).
Afterwards, chemical etching is used to remove copper left unprotected by the resist. The remaining copper constitutes a desired pattern of conductive paths on the substrate. After the remaining resist is chemically removed, an insulating overlay is subsequently laminated to the surface, the overlay being of the same material as the base.
In an alternate process described in Chapter 9 of the aforementioned book, the camera negative is used to form the layout of conductive paths. In this process, a photoresist is applied to the entire copper surface. Afterwards, ultraviolet light is passed through the camera negative to "fix" the photoresist material to the base in those areas where conductive paths are to be formed. Unprotected areas of the copper layer are then chemically etched, followed by lamination with an insulating overlay, as described above.
Despite the significant advantages FPW circuits have realized in small scale applications (e.g., cameras, telephones, personal computers, disk drives, and automobile dashboards), present technology does not provide for the creation of large-scale FPW systems which can replace RWH. For example, screen printing and photography techniques similar to those described above are currently used to produce flexible printed circuits in a batch fashion. Units, or panels, of limited size (e.g., two feet square) are typically processed one at a time because this is roughly the largest size the image-producing step (e.g., screen printing or photography) can handle easily. Further, current image-production is fairly expensive or slow to alter, resulting in high overhead costs and delays for the small number of runs which would be associated with large-scale FPW.
Because the size limitations associated with FPW have been recognized and accepted by industry, technology in this area has focused on maximizing the use of the available space on a size limited substrate. For example, U.S. Pat. No. 4,587,719 discloses a method of batch fabricating flexible circuits in an effort to provide area efficient use of a size limited substrate. As disclosed therein, orthoganol lines of conductors are joined and folded about axes parallel to the conductors. Although the disclosed method permits flexible arrays to be fabricated, its use of traditional imaging techniques imposes size constraints on the overall circuit layout design which prohibit its use as an RWH.
Further, U.S. Pat. No. 3,819,989 relates to a printed wiring multiple circuit board assembly wherein printed circuit boards are arranged at right angles to a flexible main tape 10. The flexible main tape is formed with a plurality of conductors and tape tabs 12 through 15 using conventional techniques. The tape tabs connect the main tape 10 and a plurality of overlays 16 through 19. The overlays are formed on each side of the main tape and are bonded to conventional rectangular circuit boards situated in mutually parallel planes perpendicular to the main tape. However, like U.S. Pat. No. 4,587,719, the system of U.S. Pat. No. 3,819,989 fails to disclose fabrication techniques which would permit the manufacture of RWH replacements.
Techniques aimed at joining sectors of FPW to produce the equivalent of an RWH (i.e., a "step and repeat" process) have also been attempted. However, these techniques increase manufacturing complexity and cost, and do not provide a cost effective three-dimensional breakout routing (i.e., branching) capability necessary for RWH replacement. Although individual and unique FPW segments could be combined to produce breakouts suitable for three-dimensional routing, such a combination would significantly increase the number of interconnections, further increasing design and manufacturing complexity and decreasing system reliability.
To address some of the drawbacks associated with the aforementioned step and repeat processes, U.S. Pat. No. 3,712,735 discloses a photoresist pattern controlled continuous strip etching apparatus. Here, a continuous loop photographic film transparency of a master pattern is used for contact photo printing of the pattern onto a moving resist coated strip. Because this apparatus merely addresses the problems associated with step and repeat processes by increasing the size of the master film transparency, many of the aforementioned drawbacks associated with small scale FPW applications are incurred. For example, circuits produced by the disclosed apparatus are limited to the size of the master film transparency and are not readily adapted to in-line modifications of the conductor patterns once the master film transparency has been produced.
Thus, while FPW has many recognized advantages relative to RWH, the size limits of FPW have rendered its use in replacing RWH impractical.